A blue light emitting diode (hereinafter referred to as LED) used as a light source of full color display or a signal lamp and a blue laser diode (hereinafter referred to as LD) used as a light source of a highly fine next-generation DVD that continuously oscillates at room temperature are attracting people's attention because it has recently become possible to obtain them by lamination of GaN-based compound semiconductor on a sapphire substrate.
Conventional blue semiconductor light emitting devices of this type (which means a color from ultraviolet rays to around yellow; the same applies hereinafter) are obtained by successive lamination of Group III nitride compound semiconductor (GaN-based compound semiconductor) on a sapphire substrate 71 by means of the metal organic chemical vapor deposition (hereinafter referred to as MOCVD), and are constructed in such a manner that a GaN buffer layer 72, an n-type GaN layer 73, an n-type stress-alleviating layer 74 made of In0.1Ga0.9N, an n-type cladding layer 75 made of Al0.2Ga0.88N, an n-type optical wave guide layer 76 made of GaN, an active layer 77 made of a multiple quantum well structure of InGaN-based compound semiconductor, a p-type optical wave guide layer 78 made of p-type GaN, a p-type first cladding layer 79 made of p-type Al0.2Ga0.8N, a p-type second cladding layer 80 made of Al0.12Ga0.88N, and a contact layer 81 made of p-type GaN are successively laminated, and a portion of the laminated semiconductor layers is etched by dry etching or the like, as illustrated in FIG. 47, to expose the n-type GaN layer 73 on the surface of which an n-side electrode 83 is formed, and a p-side electrode 82 is formed on the aforesaid contact layer 81.
Further, in the second ICNS (International Conference on Nitride Semiconductor) held in Tokushima, Japan in 1997, a semiconductor light emitting device having a structure is reported in which GaN-based compound semiconductor layers are laminated on a substrate with the use of 6H-SiC. However, the lamination structure of gallium nitride (GaN) compound semiconductor is the same as the aforesaid structure, the only difference being in the substrate.
Such a Group III nitride compound semiconductor for a blue short-wavelength semiconductor light emitting device is thermally and chemically extremely stable and highly reliable, and has extremely excellent properties in view of increasing its life. However, since it is stable, it must be grown at an extremely high temperature such as about 1000° C. in order to obtain semiconductor layers having a good crystallinity, as shown in Japanese Patent Gazette No. 2713094. On the other hand, with regard to semiconductor layers containing In (indium) such as an active layer, mixed crystals of the element In and GaN are not easily formed, and also the vapor pressure of In is high, so that crystals can be stacked only at a temperature of about 700° C. or less if sufficient In is to be introduced. Therefore, the temperature cannot be raised to a high temperature needed for obtaining a semiconductor layer having an excellent crystallinity, so that it is not possible to obtain semiconductor layers having a good crystallinity, raising a problem of reduction in the light-emission efficiency or deterioration in the life characteristic.
Further, a semiconductor laser constructed with AlGaN/InGaN/GaN based semiconductor has a drawback as an important physical property. Namely, the InGaN/GaN system is a lattice-mismatched system, so that an internal electric field (piezoelectric field) is always generated in the InGaN active layer by stress. In particular, InGaN materials have a physical property intrinsic to the materials such that the piezoelectric field is intensely generated. If this internal electric field is strong, electrons and holes are spatially separated, thereby reducing the recombination probability and raising the threshold value of the semiconductor laser. For this reason, reduction of the threshold value is achieved by doping the InGaN active layer with Si or the like to generate a Coulomb potential shield effect for reducing the internal electric field. On the other hand, if it is doped with an impurity, it is not possible to avoid generation of a non-light emitting recombination center, so that carriers are consumed in a process other than the light emission, thereby conversely raising the threshold value and inviting the temperature rise of the element during light emission. This imposes an obstacle in improving the life of the element, particularly in the improvement of life at the time of producing a high output. Therefore, in semiconductor lasers, doping of active layers must be avoided, so that the threshold value cannot be lowered by doping.
As described above, the InGaN materials used for an active layer of a conventional blue semiconductor light emitting device have a problem that the threshold value tends to rise by the stress accompanying the lattice mismatch. On the other hand, if mixed crystals of In with GaN are made, the lattice constant will be smaller, whereas if mixed crystals of Al with GaN are made, the lattice constant will be larger. Therefore, in a blue semiconductor light emitting device having a structure such that an active layer made of InGaN is sandwiched between cladding layers made of AlGaN, this stress cannot be eliminated.
Further, most of the apparatus for growing Group III nitride compound semiconductor layers that do not contain In are vacuum apparatus, so that continuance of crystal growth while keeping the temperature around 1000° C. imposes a heavy load on the apparatus, and also failure such as leakage is liable to occur often, raising a problem that it is extremely difficult to stably operate the apparatus.
Further, since the Group III nitride compound semiconductor is stable, it is extremely difficult to perform wet etching with chemicals, and in particular, it is not possible to build an internal electric current constriction layer therein which is needed in constructing a laser element. Also, the etching for forming a mesa-type shape must be a physical etching such as reactive ion etching (RIE), raising a problem that it is extremely difficult to form it into a semiconductor laser structure as a process.
Therefore, the inventors of the present invention have attempted to produce a blue semiconductor light emitting device using an oxide compound semiconductor. It is known in the art that ZnO, which is one of the oxide compound semiconductor, can be epitaxially grown at a temperature lower than about 600° C. by using the laser MBE method or the like, and is soluble in an alkali solution, so that the wet etching can be performed, as described in Phys. Stat. Sol., Vol. 202 (1997), pp. 669-672. However, this ZnO has a band gap of 3.2 eV, so that if this material is used as it is in an active layer, only the light emission in an ultraviolet region around 370 nm can be achieved. In order to use it, for example, as a light source of a highly fine DVD, both the transmissivity of an optical disk substrate and the recordation density onto a disk must be satisfied, so that the wavelength region of the light source is required to be within the range from 400 to 430 nm, as described in Functional Materials, Vol. 17 (1997), No. 8, pp. 18-25. In other words, as illustrated in FIG. 46, if the wavelength is shorter, the transmissivity of the optical disk substrate is greatly reduced, so that the wavelength of light is required to be larger than 400 nm because transmissivity of 75% or more is needed. Also, if the wavelength is longer, the recordation density is reduced. Due to the need in the recordation density that 15 GB or more is required on one surface of a disk in a highly fine DVD, the wavelength is required to be 430 nm or less.
On the other hand, a wider band gap of ZnO materials is achieved by forming mixed crystals of ZnO and MgO, as described in Applied Physics Letter (Appl. Phys. Lett.). Vol. 72 (1998), No. 19, pp. 2466-2468, or Material Society Forum (Mat. Sci. Forum), Vols. 264-268, pp. 1463-1466, 1998, or the like. However, a concrete method of narrowing the band gap of ZnO is not yet known in the art.
The present invention has been made in view of these circumstances, and an object of the present invention is to narrow the band gap of ZnO materials and to provide a semiconductor light emitting device with improved light-emission characteristics by using an oxide semiconductor having few crystal defects and being excellent in crystallinity as a material for an active layer of a semiconductor light emitting device such as a blue light emitting diode or a blue laser diode in which the active layer is sandwiched between cladding layers.
Another object of the present invention is to provide a blue semiconductor laser such as used in a light source of a highly fine DVD.
Still another object of the present invention is to provide a light emitting device such as a semiconductor laser in which the formation of a mesa-type shape or an internal electric current constriction layer (electric current restricting layer) is facilitated by constructing laminated semiconductor layers with oxide semiconductor capable of being subjected to wet etching.
Still another object of the present invention is to provide a semiconductor light emitting device in which an electrically conductive material is used as a substrate and electrodes can be taken out from both upper and lower surfaces.
Still another object of the present invention is to narrow the band gap of a ZnO-based compound semiconductor and to provide a semiconductor light emitting device using the ZnO-based compound semiconductor.
Still another object of the present invention is to provide a semiconductor light emitting device having a structure such that blue light is emitted without the use of an InGaN-based compound semiconductor in an active layer and stresses accompanying the lattice mismatch are not imposed on the active layer.
Still another object of the present invention is to improve the crystallinity or the electric conductivity of oxide semiconductor layers by growing each layer with good crystallinity or by improving the lamination structure, the electrode structure, or the like and to improve the efficiency of taking out the light to the outside (external differential quantum efficiency) to improve its light-emission characteristics in the case where the light emitting device is formed with the use of a znO-based compound semiconductor.
Still another object of the present invention is to provide a semiconductor laser with high characteristics in which an electric current constriction layer is effectively buried in the inside by utilizing the wet etching property of a ZnO-based oxide semiconductor.